The clip shows how, through a mastery of the science of ballet, a great dancer makes it her own. From the first moment to the last, Kirkland is absolutely in control, technically speaking, and so free to shape her musical phrasing as she chooses. Very few Giselles would choose to take the variation this slowly, but Kirkland fills up every bar with detail. Watch the luxuriance of that first posé (step-out) into arabesque on pointe at 0:06 (repeated at 0:18), and the way the working leg rises as the supporting foot rolls smoothly down through the heel. A momentary fondu, a "melting" bend of the supporting knee (as opposed to fondue, a melting of Gruyère cheese) and she's into the next step.

Giselle, created in 1841, is a Romantic-era ballet, and these are usually danced "after the beat", as if in answer to the music. Here, Kirkland combines this responsiveness with a subtle rubato (or "bending") of the score – watch the way, for example, she borrows a beat at 0:12 and expends it on that lovely airy balance at 0:13 – so that what emerges is a kind of conversation between her and the piano. Watch how she sustains those pirouettes in attitude at 0:31 and 0:38, and note the part played by her arms. If you look at her left arm in particular, you can see how she uses it to propel herself through the turn, while never interrupting its lyrical flow.

And then, after the arabesque at 0:41, she seems to step outside the dance for a moment, to check herself. This is a very Gelsey detail; it's as if, in all the sunny joy of the dance, she had an instant of foreboding, a premonition. A split second later and it's gone, whirled away in the next pirouette. The diagonal of hops from 1:00 to 1:20, as the working leg describes a series of ronds de jambe, is a perfect illustration of "after the beat" Romantic dancing, and then she's into that helter-skelter series of piqué turns, skirt flying. But, as the final pirouette to the knee demonstrates, still in exquisite control.

The years that followed the filming of this clip would not be happy ones for Kirkland, and her career would implode in a nightmare of drugs and self-harm, unsparingly detailed here. She remains, however, a much-loved legend of American ballet, and this clip shows exactly why.